Extraction electrode geometry for a calutron

ABSTRACT

The present invention relates to an improved geometry for the extraction electrode and the ground electrode utilized in the operation of a calutron. The improved electrodes are constructed in a partial-picture-frame fashion with the slits of both electrodes formed by two tungsten elongated rods. Additional rods are used to establish equipotential surfaces over the rest of the front of the ion source.

United States Patent Veach et al.

[ 1 Sept. 23, 1975 EXTRACTION ELECTRODE GEOMETRY FOR A CALUTRONInventors: Allen M. Veach; William A. Bell,

.lr., both of Oak Ridge, Tenn,

Assignee: The United States of America as represented by the UnitedStates Energy Research and Development Administration, Washington, DC.

Filed: Apr. 16, 1974 Appl. No.: 461,472

U.S. Cl 250/298; 250/281 Int. Cl. H01J 39/34 Field of Search 250/298,299, 300, 398,

References Cited UNITED STATES PATENTS Shelton et al 313/359 3,6l0,92311/1971 Bell et al 250/298 Primary Examiner]ames W. Lawrence AssistantExaminer-B C. Anderson Attorney, Agent, or Firm-Dean E, Carlson; David SZachry; Louis M. Deckelmann 2 Claims, 3 Drawing Figures US Patent Sept.23,1975 Sheet 1 of2 3,908,123

US Patent Sept. 23,1975 Sheet 2 of2 3,908,123

EXTRACTION ELECTRODE GEOMETRY FOR A CALUTRON BACKGROUND OF THE INVENTIONThis invention was made in the course of, or under, a contract with theUnited States Atomic Energy Commission.

ln all prior charge particle acceleration systems the source electrodegeometry (which influences the formation of equipotential surfaces) hasbeen critically shaped in order to achieve acceptable ion or electronbeam focus. In the calutron (electromagnetic) separation of isotopes atthe Oak Ridge National Laboratory, it has long been desirable to bringabout a reduction of the un-ionized charge materials (neutrals) in theimmediate vicinity of the source and electrodes. The reasons behind thisobjective are as follows:

1. The total useful output from a source is lowered because chargeexchange of the primary beam occurs with the neutral atoms that make upthe high pressure region existing in the electrode system.

2. High neutral particle densities in this region contribute tocontamination by scattering the primary beam.

3. An optimized electric field gradient cannot be achieved in ahigh-pressure region due to high voltage breakdown. This is especiallydetrimental when charge feed is of a reactive nature (the vapors areeasily ionized, such as those of rubidium, potassium, lithium, cerium,etc. When high voltage breakdown occurs with these elements, sustainedelectron drains to the ion source literally melt source components.

Thus, there exists a need for an improved calutron extraction electrodegeometry such that the calutron can be operated in a more efficientmanner to produce an increased ion output therefrom than that which ispossible with prior-art calutrons and at the same time providing formore stable operation of the calutron. This need has been met by thepresent invention in a manner to be described hereinbelow.

SUMMARY OF THE INVENTION It is the object of the present invention toprovide an improved extraction electrode geometry for a calutron whereina substantial reduction of un-ionized charge materials in the immediatevicinity of the source and electrodes is effected to provide anincreased ion output therefrom and more stable calutron operation.

The above object has been accomplished in the present invention byreplacing the conventional negative extraction electrode and the groundextraction electrode of a calutron with new electrodes constructed in apartial-pieture-frame fashion with slits of both electrodes formed bytwo tungsten rods. Additional parallel, spaced-apart rods are alsoprovided in each of the electrodes for the purpose of establishingequipotential surfaces over the rest of the front of the calutron ionsource. The use of these tungsten rods substantially reduces the neutralparticle density in the source region such that in the operation of thecalutron the total useful output therefrom is substantially increased.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an oblique front view of thenew ground extraction electrode.

FIG. 2 is an oblique front view of the new negative extractionelectrode.

FIG. 3 is a cross-sectional view of the new extraction electrodes inrelation to the calutron ion source exit slit.

DESCRlPTlON OF THE PREFERRED EMBODIMENT Only the necessary portions ofthe improved extrac tion electrode geometry and its relation to thecalutron ion source are shown in the drawings for an understanding ofthe present invention. It should be understood that the improvedextraction geometry of the present invention may be utilized in acomplete calutron system such as disclosed in US. Pat. No. 2,709,222 toE. 0. Lawrence, to which reference is made. It should also be understoodthat the necessary charge vapor for the ion source of the presentinvention is supplied thereto in a conventional manner. One means forheating the charge material for use in the ion source is disclosed inUS. Pat. No. 3,115,575 to W. A. Bell et al., to which reference is made.

Referring now to the drawings, the ground electrode of the presentinvention is illustrated in FIG. 1. Paral lelly mounted in a graphiteframe are a plurality of spaced-apart elongated metal rods 3 which maybe three thirty-seconds inch in diameter 7 inches long and may beconstructed from tungsten, for example. The frame 1 is open at thebottom thereof and is provided with an upper extension 2, only partiallyshown, which is utilized for mounting this electrode in front of thenegative electrode in spaced relation thereto as schematically shown inFlG. 3. The space 4 between a pair of the rods 3 serves as the groundelectrode aperture or slit through which the ions are withdrawn from thecalutron ion source.

The negative electrode of the present invention is illustrated in FIG. 2of the drawings. Parallelly mounted in a graphite frame 5 are aplurality of spacedapart elongated metal rods 7. The frame 5 is open atthe bottom thereof, as shown. The rods 7 may be three thirtyseconds inchin diameter by 7 inches long and may also be constructed from tungsten,for example. The frame 5 is provided with an upper extension 6, onlypartially shown, which is utilized for mounting this electrode betweenthe ground electrode 1 and the calutron ion source 10 in spaced relationtherebetween as schematically shown in FIG. 3. The frame 5 is alsoprovided with a pair of flanges 5' in which are mounted a pair ofelongated rods 7'. The space 9 between the rods 7' which is inline-of-sight with the space 8 between a pair of electrodes 7, as moreclearly seen in FIG. 3, defines a slit or aperture for the negativeelectrode which is in alignment with the slit 4 of the ground electrode,and ions from the calutron ion source 10 are withdrawn by the electrodes5 and 1 from the arc chamber 11 through its exit aperture or slit 12which is in alignment with the respective slits 9, 8, and 4 of theelectrodes 5 and 1. The ions thus withdrawn from the calutron ion source10 are electromagnetically separated by the calutron magnetic field,which is perpendicular to the plane of the paper in FIG. 3, before theyare received by respective collection pockets of the calutron receiver,not shown, in a conventional manner as described in the prior artmentioned hereinabove. It should be understood that the negativeelectrode 5 is connected to a source of negative voltage and theelectrode l is connected to ground in a conventional manner.

An important feature of the tungstcirrod electrodes of the presentinvention is that they reduce the area which is available for thecollection of reactive elements. Such collected materials are well knownfrom past experience to be liberated by heat from beam and high voltagebreakdown (sparking), thus causing E X Pi drains to avalanche in thesource electrode region. This problem of avalanching drain has long beenobjectionable in source operation because of the off time associatcdwith the high drain condition. Additionally, contamination in theseparated isotopes is increased due to this type of sparking and highvoltage kickoff." in extreme drain conditions, the source voltage islowered because the drains exceed the limits of the power supply.

The use of tungsten rods in the extraction electrodes, as describedabove, substantially reduces the neutral particle density in the sourceregion wherein in the operation of the calutron the total useful ionoutput there' from is substantially increased, which can be seen fromthe operating data which will now be described.

The present tungsten-rod extraction electrodes were initially tried forsilicon isotope separation. The charge material used in the separationwas SiS (silicon disulfide). In a 24l-hour comparison involving 14different runs, the average total silicon output for the standardelectrodes was 18.88 milliamperes as compared with 38.39 milliamperesfor the tungsten-rod electrodes. This was an increase of 203% over theoutput from the standard-electrode runs.

The most impressive difference in the performance of the two electrodegeometries was illustrated in the rubidium separation. Rubidium issecond only to ccsium in having the lowest ionization potential of thestable elements. Drain problems and high voltage kickoff areunpleasantly common and contribute greatly to the contamination of theseparated isotopes in priorart calutrons. The rubidium vapors areabsorbed by the graphite source parts and the conventional graphiteelectrodes. Heat and sparking liberate the rubidium into the electroderegion to elevate the pressure, and drains are high in conventionalcalutron operation. The output (weighted average) of eight calutronsused in standard prior-art rubidium separations was determined to be 996milliamperes. When the new electrode system was installed on an ionsource, the average ion output during 154 hours of run time was 38.96milliamperes, an increase of almost 400%.

All of the sources utilized in the rubidium separation were thenequipped with the present new type of electrode. After thousands ofoperating hours in eight separators, the average rubidium ion output was9.94 milli amperes using standard electrodes and 28.79 milliamperesafter the sources were equipped with the newtype electrodes. An ionoutput increase of 290% over the standard electrode system was thusobtained for the rubidium series.

The increase in output achieved in the present invention is attributedlargely to decreasing the pressure in the source and electrode slitregion, as evidenced by the following argument. Any ion from the sourcethat experiences a charge exchange event will be registered as a portionof the metered source drain but will not be monitored at the receiver ascollected output, Therefore, if ions were being lost by this process,the ratio of the collected output to source drain in the standardelectrode system should be smaller in value than for the new electrodegeometry. This ratio for the old geometry was t), 152 as compared to0.288 for the present new system. Thus, the ratio for the presenttungsten-rod electrode system was almost twice l.9l the value derivedfrom the standard electrode system.

The increase in ion output is somewhat higher than would be expectedfrom the preceding charge exchange process alone. At high pressure (5 Xit)" torr) an annular glow around each of the tungsten rods is visible.The axis of the rods is parallel to the magnetic field, and with theelectric field gradient there is an electron trap which the primary beammust pass through. This added source of electrons for beamneutralization could also have been partially responsible for theincrease in collected current, since improved neutralization wouldreduce the amount of ion loss in a radial direction to the bafflesbecause of space charge effects. This effect could also possibly be usedto explain why the electrode geometrical shape is not required to be ascritical as current theory predicts.

Another point in favor of the present invention is that the down time"because of drain problems and high voltage kickoff is reduced by the useof the present new electrode geometry. Electrical timers were used toread when the high voltage was not on during the run; likewiseelectrical counters recorded the number of times the high voltage waskicked out during the run. The new electrode system showed ahigh-voltage down time of only l.5% and the high voltage kicked off" atthe rate of 4.67 times per hour during this same time period With thestandard electrode geometry, the down time was 5.2% and the high voltagekicked off an average of 36 times per hour. In each instance, highvoltage cycling is detrimental to isotopic purity.

The new extraction electrode geometry is presently being utilized in ahigh purity Te run and its use will soon be scheduled in a second-passtin run. It should be understood that the new electrode geometry of thepresent invention is not restricted for use in separation runs involvingthe above-mentioned charge materials, but will be substantially equallyeffective for use with all types of charge materials includingparticularly those of a reactive nature such as potassium, lithium,cerium, etc.

This invention has been described by way of illustration rather than bylimitation and it should be apparent that it is equally applicable infields other than those described.

What is claimed is:

l. in a calutron system including an ion source chamber provided with aside chamber cover having a narrow elongated ion exit slit, said chamberadapted to receive a vaporized charge material therein, means forproducing an arc discharge which is associated with and passes throughsaid chamber by means of arc defining slots for ionizing the chargematerial within said chamber, a magnetic field for effecting chargesepara tion of the ions as they pass from said ion source exit slit, theimprovement comprising an improved extraction electrode geometryincluding a negatively biased graphite accelerating electrode mountedadjacent to and parallel with said ion source chamber cover, saidnegative electrode being in the form of a first bottomopened pictureframe provided with a first plurality of spaced-apart metal rods mountedin parallel relation in the vertical legs of said frame, said framebeing provided with a pair of flanges extending rearwardly from aboutthe center of said frarne's vertical legs, a pair of spaced-apart metalrods mounted between and to said flanges, said pair of rods defining anion exit slit in alignment with said ion source cover exit slit and inalignment with a slit formed between two of said first plurality ofmetal rods; and a graphite ground electrode mounted adjacent to andparallel with said negative electrode, said ground electrode being inthe form of a second bottom-opened picture frame provided with a secondplurality of spaced-apart metal rods mounted in parallel relation in thevertical legs of said second frame with two of said second plurality ofrods defining an ion exit slit in alignment with all of said previouslymentioned exit slits, whereby during operation of said calutron all ofsaid metal rods except those used for providing said ion exit slits areutilized to establish equipotential surfaces over the rest of the frontof said ion source for effecting a substantial reduction of unionizedcharge materials thereabout, thereby providing an increased ion outputfrom said ion source and more stable calutron operation.

2. The calutron set forth in claim I, wherein all of said metal rods aretungsten.

1. In a calutron system including an ion source chamber provided with aside chamber cover having a narrow elongated ion exit slit, said chamberadapted to receive a vaporized charge material therein, means forproducing an arc discharge which is associated with and passes throughsaid chamber by means of arc defining slots for ionizing the chargematerial within said chamber, a magnetic field for effecting chargeseparation of the ions as they pass from said ion source exit slit, theimprovement comprising an improved extraction electrode geometryincluding a negatively biased graphite accelerating electrode mountedadjacent to and parallel with said ion source chamber cover, saidnegative electrode being in the form of a first bottom-opened pictureframe provided with a first plurality of spaced-apart metal rods mountedin parallel relation in the vertical legs of said frame, said framebeing provided with a pair of flanges extending rearwardly from aboutthe center of said frame''s vertical legs, a pair of spaced-apart metalrods mounted between and to said flanges, said pair of rods defining anion exit slit in alignment with said ion source cover exit slit and inalignment with a slit formed between two of said first plurality ofmetal rods; and a graphite ground electrode mounted adjacent to andparallel with said negative electrode, said ground electrode being inthe form of a second bottom-opened picture frame provided with a secondplurality of spaced-apart metal rods mounted in parallel relation in thevertical legs of said second frame with two of said second plurality ofrods defining an ion exit slit in alignment with all of said previouslymentioned exit slits, whereby during operation of said calutron all ofsaid metal rods except those used for providing said ion exit slits areutilized to establish equipotential surfaces over the rest of the frontof said ion source for effecting a substantial reduction of un-ionizedcharge materials thereabout, thereby providing an increased ion outputfrom said ion source and more stable calutron operation.
 2. The calutronset forth in claim 1, wherein all of said metal rods are tungsten.